(1) Field of the Invention
The present invention relates to the improvement of flat wound electrode assemblies for the purpose of improving the volume energy density of cells.
(2) Description of the Prior Art
In recent years, there has been strong demand for the improvement of the energy density and high-rate discharging characteristics of cells.
By using wound-type electrode assemblies the surface areas of the negative electrode and positive electrode have been increased, thus trying to improve the energy density and high-rate discharging characteristics of cells. On the other hand, square cells and cells using laminate outer casings can be efficiently mounted in equipment such as mobile phones and notebook personal computers. In these applications, there is strong demand in the market for thin cells having high energy density.
In square cells and cells using laminate outer casings, flat wound electrode assemblies are usually encased rather than circular ones. Such electrode assemblies are prepared by such a method as shown in FIG. 5. A winding core 1 having a circular cross section and a slit 2 in its diameter direction is used. A strip-like separator 3 is passed through the slit and an negative electrode 4 and positive electrode 5 are engaged on each surface of the separator while it is being wound, resulting in a circular wound electrode assembly. After removing the winding core 1, the electrode assembly is pressure-molded into the flat would electrode assembly as shown in FIG. 5(e).
However, electrode assemblies produced by this method have the separator laminated in quadruplicate in the center of the electrode assemblies as shown in FIG. 5(e). The excessive triplicate separator portion that does not contribute to insulation increases the thickness of the wound electrode assembly, causing the problem of an increased volume and reduced energy density.
Previously suggested methods of producing wound electrode assemblies include the following techniques.
(1) There is suggested a technique in which by changing the shape of the winding core in a cylindrical cell having a wound electrode assembly, it is made easy to insert a welding electrode bar for welding the outer casing can and collector, so as to avoid defectiveness caused by electrode bar insertion (see, e.g., Patent Document 1).
(2) There is suggested a technique in which by changing the shape of the winding core in a cylindrical cell having a wound electrode assembly, it is made easy to remove the winding core from the wound electrode assembly with the winding core (see, e.g., Patent Document 2).
(3) There is suggested a technique in which: the both surfaces and ends in a longitudinal direction of an electrode plate of one polarity is enclosed with a separator; via the separator, a tip of the other polarity electrode plate is placed near the first folded part of the one polarity electrode plate, the placement being inside the end to the first folded part of the one polarity electrode plate; the other polarity electrode plate is folded at the end of the one polarity electrode plate, and then wound to face the one polarity electrode plate via the separator. Thus, different polarity electrodes face each other at the entire winding core, improving the volume energy density of a square cell (see, e.g., Patent Document 3).
Patent Document 1: Japanese Unexamined Patent Publication No. 03-141558.
Patent Document 2: Japanese Unexamined Patent Publication No. 11-307132.
Patent Document 3: Japanese Unexamined Patent Publication No. 09-213374.
According to Patent Document 1, there is a step in which an auxiliary pin is removed in advance, and then the winding core is revolved against a group of wound electrodes, so as to press the separator sandwiched between the winding core and auxiliary pin against the inside wall of the group of wound electrodes. This step causes to obtain a large cylindrical cavity, thus avoiding defectiveness caused by electrode bar insertion. This technique, however, allows only for cylindrical wound electrodes; as shown in the figures of this document, the winding of both electrode plates commences right after insertion of the group of wound electrodes and separator prepared, and therefore the both electrode plates face each other via the separator in the center of thus wound article. This causes, at the time of molding into a flat shape, the electrode plate at the center of the wound article to be folded acutely, and the resulting edged portion to break through the separator, resulting in short circuiting between the electrodes. Thus, the problem of cell reliability remains with this technique. Additionally, since the electrode plates are inserted right after the separator is inserted into the winding core and the winding has commenced, it is impossible to greatly reduce the length of the separator at the center of the wound article, compared with a winding core that is so divided that its cross section is symmetrical.
According to Patent Document 2, with the winding core comprising the winding core body and a pressing pin, it is easy to remove the wound article from the winding core after completion of the winding. This technique, however, has such a complicated mechanism that the separator is inserted into the winding core body and the pressing pin is rotatively lowered from above the winding core body. Such complicated mechanism is not appropriate for rapid mass production of wound electrode assemblies.
According to Patent Document 3, it is possible to improve volume energy density. This technique, however, requires the following complicated steps: two separators placed in both sides of a negative electrode plate are adhered and then cut with a separator adhesion and cutting device, and then the negative electrode is inserted further inward between the two adhered separators; the resulting article with the negative electrode plate therebetween is moved between winding cores; subsequently, a positive electrode is inserted between the winding cores from the direction opposite the negative electrode; the winding cores are so revolved that they sandwich the positive electrode plate, separators, and negative electrode plate, with the positive electrode plate being on the innermost side; before further revolution, the positive electrode plate is cut to a predetermined length with a positive electrode cutting device, and the negative electrode plate is cut to a predetermined length with a negative electrode cutting device; and one of the separators is heat welded to the separator junction portion of the other already wound separator. Additionally, an edged portion can break through the separators, in the same manner as Patent Document 1. Thus, this technique has the problem of poor work efficiency, increased manufacturing cost, and poor reliability.